1. Field of the Invention
This invention relates to the polymerization of vinylphosphonic acid with the aid of a free radical-forming catalyst.
Polyvinylphosphonic acid is of great practical interest, for example for protection from corrosion, flameproof finishes, the production of printing plates and as an additive to washing and cleaning agents.
2. Description of the Prior Art
Polyvinylphosphonic acid has hitherto been prepared according to Examples 7 and 8 of German Pat. No. 1,106,963 with the aid of aprotic solvents, preferably ethyl acetate. It is thereby obtained as a precipitate and must be separated off, for example from ethyl acetate and impurities. The polyvinylphosphonic acid thus obtained still contains about 10% of free vinylphosphonic acid. On filtration by suction, safety measures are necessary, and solvent losses arise. Polyvinylphosphonic acid, which in some cases is nevertheless considerably contaminated by by-products, is also formed when vinylphosphonic acid is polymerized according to Examples 1 and 6 of the same Patent in the absence of solvents. This procedure initially leads to very highly viscous reaction products which are difficult to agitate and finally become solid, and these can no longer be stirred or kneaded. In practice, this procedure therefore cannot be carried out.
A preparation process has thus been sought for vinylphosphonic acid, in which
1. filtration with suction can be omitted, PA0 2. the mixture can be stirred continuously, PA0 3. only a small residual monomer content remains at the end, PA0 4. the reaction products can be further diluted directly, for example with water, and PA0 5. the process can be carried out in water or other protic, water-miscible diluents. PA0 1. dissociate by half even below 122.degree. C. within one hour and PA0 2. belong to the groups of compounds comprising aliphatic peroxy-esters, aliphatic diacyl peroxides or aliphatic azo compounds.
It can be seen from Examples 3, 4 and 5 of German Pat. No. 1,106,963 that the polymerization of vinylphosphonic acid can be carried out in relatively large amounts of protic solvents (50% of isopropanol or 100% of water) using relatively short polymerization times of only 5 or 6 hours and in the presence of only 0.5% of free radical initiators (di-benzoyl peroxide or potassium persulfate), which are added at the start.
On the basis of a certain increase in viscosity of the resulting reaction solutions, it is assumed that solutions of polyvinylphosphonic acids are also formed under these conditions. No concrete information is given relating to the yield of polyvinylphosphonic acid or the residual monomer content. This cannot be surprising, since the P.sup.31 -NMR spectroscopy which today permits unambiguous determination of non-polymerized monomeric vinylphosphonic acid, in addition to polyvinylphosphonic acid, was not yet available at that time.
Duplications of Examples 3, 4 and 5 of the above Patent and subsequent investigations of the reaction solutions by P-NMR spectroscopy show that the yields of polyvinylphosphonic acid under the experimental conditions disclosed in these Examples is in fact only about 35-40%, or about 6-10%, based on the vinylphosphonic acid employed. At no point in the Patent are there indications of other polymerization conditions which could lead to higher contents of polyvinylphosphonic acid in protic diluents.
In a publication written by the authors of German Pat. No. 1,106,963 (Angew. Chemie 1962, page 974), it is likewise generally declared, without limitation, that vinylphosphonic acid is a compound which readily polymerizes and the homopolymerization of which can easily be initiated. On the basis of these prior publications, the expert must have formed the impression that vinylphosphonic acid can be polymerized in protic diluents in an analogous manner to in aprotic diluents, in good yields and under conditions which require relatively little energy and are specific.